1. Field of the Invention
This invention relates to ion atomic clocks and frequency standards.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
For its atomic clock characterization activities, the Naval Research Laboratory (NRL) requires reference clocks with better stability than those being characterized. Traditionally this has been accomplished using hydrogen maser atomic clocks, but recently the stability of the clocks being characterized has approached, or even exceeded, that of the masers. Thus it is necessary to use a reference clock with better stability, particularly on long time scales, than a hydrogen maser. An analogous situation exists in the Jet Propulsion Laboratory's (JPL's) Frequency Standards Test Lab (FSTL). This facility is charged with characterizing hydrogen masers that will be used in the National Aeronautics and Space Administration (NASA) Deep Space Network and so a reference clock with better long-term stability than a maser is needed in the FSTL as well.
While there are notable exceptions, typical masers drift at the 5×10−16 to 2×10−15/day (d) level [1]. In 2002, the first operational multi-pole trapped ion clock (or Linear Ion Trap frequency Standard (LITS)-8 or LITS-8) was installed at the United States Naval Observatory (USNO) and demonstrated a drift rate of about 1×10−16/d [2]. The multipole trap greatly reduces sensitivity to frequency shifts caused by variations in ion number, but the small observed drift rate of LITS-8 was later attributed to a residual number-dependent effect [3].
In 1996, JPL developed an implementable and continuously operating mercury ion (Hg+) clock, based on a quadrupole ion trap, for operation in the Deep Space Network (DSN) (LITS 4-7)[4]. The standard could discipline several Local Oscillators and could operate with a practical Voltage Controlled Oscillator (VCO) as the Local Oscillator (LO).
In 1999, JPL introduced a specialized “multipole” trap design that greatly reduced systematic relativistic effects [5], and in 2002, JPL developed implementable and continuously operating multipole based mercury ion clocks LITS-8 and LITS-9 [2] (LITS-8 and 9 were still the size of a full rack).
In 2007-2008, JPL demonstrated exceptional long-term stability in a trapped ion clock that used a multi-pole ion trap, coupled with a second-order Zeeman compensation scheme added to LITS-9, that virtually eliminated relativistic frequency shifts due to variations in the number of ions trapped [6]. Over a 9-month period of continuous unattended operation, LITS-9 exhibited a drift of less than 3×10−17/day relative to TT(BIPM07) [3, 7], the world ensemble of primary standards, making it significantly more stable than most hydrogen masers [1].
Based on the LITS-9 stability results and lessons learned from that unit, since 2010 we have been developing the next generation mercury trapped ion frequency standard (referred to as Linear Ion Trap frequency Standard-10 (LITS-10) or L10). The immediate goal of L10 is to provide an advanced reference clock capability at the NRL and the JPL frequency standard test laboratories.